1. Field of the Invention
The invention relates to a hybrid driving apparatus in which a plurality of driving power sources are power-transmittably linked to rotating elements of a gear mechanism capable of differential motion and to wheels, and a control method thereof.
2. Description of the Related Art
In a hybrid vehicle equipped with an internal combustion engine and a motor-generator as a plurality of driving power sources, it is possible to improve fuel economy and reduce exhaust gas while making the best of the characteristics of the engine and the motor-generator. An example of such hybrid vehicles having an internal combustion engine and a motor-generator as a plurality of driving power sources is described in Japanese Patent Application Publication No. JP-A-2005-125876.
The hybrid vehicle described in Japanese Patent Application Publication No. JP-A-2005-125876 has an engine, a first motor-generator and a second motor-generator. Besides, the hybrid vehicle is provided with a power distribution mechanism to which the engine, the first motor-generator and the second motor-generator are linked. This power distribution mechanism is made up of a planetary gear mechanism whose sun gear, that is, a reaction force element, is linked to the first motor-generator, and whose carrier, that is, an input element is linked to the engine and whose ring gear, that is, an output element, is linked to a first intermediate shaft. A path from the first intermediate shaft to axle shafts is provided with a clutch. On the other hand, the second motor-generator is linked to a first intermediate shaft gear via a speed reducer. This speed reducer is made up of a planetary gear mechanism. The second motor-generator is linked to the sun gear of the planetary gear, and the ring gear thereof is linked to the first intermediate shaft, and the carrier thereof is fixed. Furthermore, a second intermediate shaft is linked to the first motor-generator, and a path from the second intermediate shaft to the aforementioned axle shafts is provided with a clutch. Then, by controlling the engagement and release of the two clutches, the sun gear (reaction force element) or the ring gear (output element) of the power distribution mechanism can be selectively linked to the axle shafts. In consequence, a state where the power transmission loss is large, such as a power circulating state or the like, is avoided, according to the aforementioned patent application. A hybrid vehicle equipped with an internal combustion engine, a first motor-generator and a second motor-generator is also described in Japanese Patent Application Publication No. JP-A-2005-155891.
However, in the hybrid vehicle described in Japanese Patent Application Publication No. JP-A-2005-1255876, there is a need to improve the power transmission efficiency in a wider range of the region of running (a control range of the speed change ratio).